1. Field of the Invention
The invention is directed to an electrolytic electrode and a coating thereon having decreased amounts of platinum group metals with little or no valve metal for use in the electrolysis of aqueous chlor-alkali solutions.
2. Description of the Related Art
Lifetimes of electrodes, composed essentially of an active coating on a substrate, are a function of both the amount of active material applied to the substrate and the current density. Decreasing the amount of coating or increasing the current density results in a more rapid failure of the electrode. In general, an early failure of an electrode is attributed to two major factors, loss of the active coating and dissolution, or in case of the film-forming metals, passivation of the substrate. Sometimes these occur simultaneously and the electrode at the end of its lifetime may show some active material left in the coating, but the substrate has passivated.
Heretofore, a common solution to the problem of loss of the active component in the coating and passivation of the substrate was use of thicker coatings, i.e., higher loadings of the active component. Thicker coatings produced by the application onto the substrate of several e.g. ten-twenty or more layers of the active coating proved beneficial for lifetimes of the electrodes with the same coating composition. Simplicity of the solution to the problem of electrode lifetimes made thicker coatings a popular and almost universal remedy. However, an increase of the coating thickness meant a significant increase in cost due to the increased amount of platinum group metal utilized in the coating, as well as increased labor costs due to the higher number of layers applied.
Many attempts have been made to economize on the precious metal content of these coatings, usually, by partly replacing the platinum-group metal oxide with a compatible non-precious metal oxide such as tin dioxide (see, for example, U.S. Pat. No. 3,776,834) or tin and antimony oxides (see, for example, U.S. Pat. No. 3,875,043).
Additionally, the use of valve metal oxides in combination with the precious metal coating has been attempted. Such an anode for use in electrolytic processes such as chlorine production is disclosed in U.S. Pat. No. 4,070,504. The electrode utilizes a titanium or tantalum metal substrate with a coating of mixed metal oxides, preferably valve metal oxides and platinum group metal oxides which have been doped with a doping oxide. The valve metal oxide is present in the coating in an amount of greater than 25 mole percent.
The problem of electrode lifetime is also important with oxygen evolving electrodes used as anodes in various industrially important electrochemical processes, e.g., low current density oxygen evolving processes. In these processes, electrodes with platinum-group metal oxide coatings are used as oxygen evolving anodes. These platinum-group metal oxide anodes are found to operate very well under relatively difficult conditions imposed by these processes (e.g. current densities of up to 2-3 kA/m2 in aggressive electrolytes). However, to attain an acceptable performance under these conditions, these electrodes must have relatively high platinum-group metal loadings (e.g. more than about 12-16 g/m2). Various tests with the known oxygen evolving anodes have shown, however, that while electrodes with platinum-group metal oxides operate with satisfaction under these conditions, they fail rapidly if the operating current density is increased to 5 kA/m2 or more. The simple approach of a higher loading, therefore, meant only higher costs but not better service life. In recent years, the rapid development of high speed plating (electrogalvanizing) techniques has amplified the problem.
It has been known from U.S. Pat. No. 3,711,385 that the electrocatalytic coating of a platinum-group metal oxide could be made as thin as 0.054 micrometers. In practice, however, it has been found that to achieve any acceptable lifetime somewhat thicker coatings were necessary. Hence, usually ten to twenty thin coatings of a suitable paint solution are applied to the film-forming metal base and heated each time to give an electrocatalytic coating formed from the decomposed component of the paint containing about 5 to 20 grams by metal of the platinum-group metal oxide per square meter of the projected electrode surface.
An oxygen evolving anode made by coating a titanium substrate with iridium oxide or iridium/ruthenium oxide using a mixture of codeposited titanium oxide or tin oxide and tantalum oxide or niobium oxide with platinum metal as the electrode underlayer has been disclosed in U.S. Pat. No. 4,481,097. The electrode active component includes 1.3 g/m2 of platinum metal in the underlayer and 3.0 g/m2 of iridium oxide in the top layer. According to the document, the electrode has maximum lifetime of 80 hours under accelerated lifetime tests performed in an aqueous solution with 150 g/l of H2 SO4 as an electrolyte at 80° C. and current density of 25 kA/m2.
It would, however, be desirable to provide an electrode for such service having improved lifetimes without this being offset by a prohibitive cost due either to a high cost of the electrode materials or a high production cost or a combination of these.